Organosilicon polymers, as a class of engineering thermoplastics, have good heat resistance and excellent mechanical properties. A precursor for polyesters, polyamides, polyimides or polyamide-imides having organosilicon components is a polyfunctional molecule having an organosilicon moiety, preferably of a polycarboxylic character which can be reacted readily with an alcohol or an amine, etc., to produce a polyester, polyamide, polyimide, or polyamide-imide, etc., of suitable chain length. Such precursors include silicon-containing aromatic polyacids such as 2-trimethylsilyl-terephthalic acid and 5-trimethylsilyl-isophthalic acid, both of which have been described in the proir art.
Despite the preparation of 2-trimethylsilyl-terephthalic acid and 5-trimethylsilyl-isophthalic acid, as noted above, prior investigators have not extensively pursued the application of these materials in polymers. Preparation of these materials by procedures described in the prior art typically resulted in products contaminated with oxidation by-products which required recrystallization steps to remove. For example, as is taught in Helv. Chim. Acta 51, 553-6 (1968) and Helv. Chim. 54, 117-35 (1971), stoichiometric oxidations of 2-trimethylsilyl-p-xylene and 5-trimethylsilyl-m-xylene with potassium permanganate in aqueous pyridene yielded 92% and 91% of 2-methylsilyl-terephthalic acid and 5-trimethylsilyl-isophthalic acid respectively. However, the reaction products contain toluic acid partial oxidation products and manganese dioxide which must be removed before further use of the products can be attempted. In addition, the use of highly corrosive potassium permanganate requires process equipment able to withstand such corrosive action and prevent added contamination of the product from corrosive attack of the process equipment.
Surprisingly, it has been found that silicon-containing acids such as 2-trimethylsilyl-terephthalic acid and 5-trimethylsilyl-isophthalic acid can be prepared by a catalytic reaction using a cobalt-manganese-bromine catalyst in the presence of oxygen as the oxidant. The instant invented process offers severl adavantages over prior art processes in (1) that the reaction is catalytic rather than stoichiometric, thus offering a raw material advantage, (2) molecular oxygen is the oxidant rather than an oxidizer such as potassium permanganate with its concurrent production of by-products by reduction of the oxidizing agent, and (3) the by-product of the catalytic oxidative process is water which is easier to remove from the catalytic reaction products than the by-products of the stoichiometric oxidation process.